The invention relates to a method of producing a metal strip having areas of different thickness over its width and also to an apparatus for the performance of the method.
In order to make an economic use of material and dimension structural members in dependence on loading, for a fairly long time structural members have been used which have a differential thickness over their surface and/or length (Shaping Techniques, 7th Aachen Steel Conference, 26-27 March 1992 "4.2 Rolling of longitudinal sections optimised for loading" by B. Hachman, R. Kopp, Aachen). The initial products of such structural members are known as "tailored blanks". In practice there are two basically different methods of producing such tailored blanks.
In a first prior art method the thickness of a strip is reduced in portions in a roll stand nip which can be moved into position. The blanks for the required structural members can then be cut out of such a strip of differential thickness. The advantages of such a procedure lie in the fact that the transitions from the minimum to the maximum strip thickness are continuous. However, it is a disadvantage that the transitions are very long, since the roll nip cannot be provided very quickly. Another disadvantage is that very considerable forces are required for bringing up the roll nip. A roll stand must be particularly designed for this purpose, for example, it must have a special compensating device for the expansions of the roll stand under these strong forces. Moreover, in that method the otherwise customary roll speed of several hundreds of meters cannot be used. Such a method is therefore unsuitable for the production of tailored blanks.
It is also known to produce a metal strip having differential thickness areas over its width by rolling into the strip by means of a pair of rolls a strip of lesser thickness extending in the longitudinal direction of the strip, one roll having in the axial direction at least one portion of larger diameter than in the remaining zone (DE 33 43 709 A1). It is not known that such a method has been adopted in practice for the production of tailored blanks.
In contrast, another method of producing tailored blanks is very widespread. The method starts from two sheets of different thickness, which are welded to one another via a butt joint. The advantages of such a method reside in the low cost of apparatus for welding together, so that the method is also suitable for small runs. The disadvantages are that there are no gentle transitions from the thinner to the thicker material, so that the saving in weight is not optimum, having regard to the loading occurring. There is also the aspect that the sudden change in thickness also produces a sudden increase in stress from the thicker to the thinner material. Lastly, due to the weld which must be produced, the production speed of such tailored blanks is low.
Finally, a shaping process is known under the name of "full section draw forming" (Journal "DRAHT" (WIRE) 18 (1967) No. 1, pages 33 to 38), wherein for the production of a strip a solid body is drawn through a nip formed by two freely rotatable or driven rolls. There is no provision for the production of strips having differential thickness areas over their width.
Starting from this point, it is an object of the invention to provide a method and an apparatus for the production of a metal strip with areas of different thickness over its width and a gentle transition, thus permitting low-cost production at a comparatively high manufacturing speed.